An air conditioning system commonly comprises a remote controller, an indoor apparatus and an outdoor apparatus. It is known that signal transmission between these apparatuses can be done with a two-wire crossover wiring system utilizing serially encoded signal transmission technology so as to reduce the number of wires used. According to this method, it is currently possible for a single controller to control a plurality of indoor and outdoor apparatuses. FIG. 5 shows a basic scheme of a conventional two-wire crossover wiring system for serial signal transmission. Also, FIG. 6 shows a conventional differential transmission circuit as an example of a serial transmission interface. In FIGS. 5 and 6, a remote controller 1' is connected to indoor apparatuses 2a' and 2b' and outdoor apparatuses 3a' and 3b' through a crossover wiring 5' so that the control and monitoring of air conditioning systems can be centralized.
Each apparatus (the remote controller 1', the indoor apparatuses 2a' and 2b', and the outdoor apparatuses 3a' and 3b') has a serial transmission interface 6' and a microcomputer 7'. When the microcomputer 7' carries out transmission and reception of signals, it controls control signals DE and RE of a differential line driver/receiver IC 8'.
In the example of FIG. 5, the two wires of the crossover wiring 5' have polarity, and line A and line B have to be connected to terminal A and terminal B at the signal terminal 4', respectively. In a conventional system, it has been a problem that, when this connection is reversed, normal signal transmission operation becomes impossible, and the air conditioners cannot be operated properly.